In computer-assisted operations, the position and orientation of surgical instruments relative to the patient's anatomical structures can be represented to the surgeon with the aid of a navigation system (e.g., on a display device of the navigation system). In addition to the anatomical structures, the instruments used during the medical procedure may be tracked using suitable reference devices (also referred to as reference systems), such as marker means. These reference systems include, for example, active or passive marker structures, and can be used to identify a position in three-dimensional space to a medical navigation system by emitting or reflecting, for example, infrared radiation.
In a registration process, the anatomical structures can be initially correlated with reference systems mounted thereon via navigable pointers. Thus, the structures can be tracked by the navigation system during the medical procedure.
The same is true for the instruments and their reference systems (e.g., marker means), wherein instrument-typical functional elements may be tracked. The functional portions can include, for example, surfaces of tools (e.g., cutting edges, tips, etc.) that may be used, for example, to treat bone structures. In using such instruments, the surgeon may employ a navigation system to obtain precise information concerning the position and orientation of the bone structures, even when visibility is impaired. The spatial position of the instrument and its functional elements can be represented on a display device, wherein the representation can be based on a correlation of stored geometric data of the instrument and the spatial data (which can be ascertained by the navigation system via the reference device attached to the instrument). When the stored geometric data, which mainly describe the functional elements of the instrument in relation to the reference device, deviate from the actual geometry of the instrument (e.g., after the instrument has been damaged), then an incorrect representation of the relationship between the actual instrument and the anatomical structure to be treated may be produced. This is particularly true if the information stored in the database concerning the initial outline of the instrument continues to be used.
Typically, the relationship between the functional element of the instrument and its reference device is defined once on the basis of manufacturing documents. Compliance of this relationship can be ensured by surveying the instrument and reference device after the manufacturing process. The relationship can be stored in the navigation system and retrieved during or before subsequent operations. These so-called pre-calibrated instruments (e.g., bore guides) are occasionally verified pre-operatively using navigable aids to confirm that they are dimensionally accurate. If they are not sufficiently dimensionally accurate, a message can be displayed to the user indicating that the instrument is out of calibration.
Matching model data to the possibly defective instrument is not possible in verification. The precision that verification may achieve is inevitably dependent on the precision of the navigation system, since the respective reference devices are again correlated with each other. Visibility problems and occasionally poor manageability when simultaneously positioning the instrument and the aid tend to extend the operation time.
In another method, a calibration procedure of the instrument may be carried out before the instrument is used. In this method, the values of functionally relevant parameters of the instrument (length, diameter, . . . ) used during navigation but still unset, or unset values for assigning the reference device to the functional element (e.g., socket driver with variable sockets) or the deviations of the actual instrument from the model stored in the database are determined by means of a navigable calibration tool. The calibration tool may be temporarily or permanently stored in the navigation system and utilized as needed. Here, too, the visibility and manageability problems described above have a negative effect on the operation time. Calibration functions well for instruments having functional elements with simple geometries, but again only within the limits of the precision that the navigation system can achieve.
Completely surveying functionally relevant structures of an instrument that is difficult to calibrate or verify represents a third calibration method. However, this method is not currently employed since suitable aids are not available, nor could the required procedures be carried out within a reasonable time frame.
Marker means that represent examples of a reference systems can be detected by means of a detection means (e.g., a camera or ultrasound detector). Such detection systems are also referred to as navigation systems and used in IGS (image-guided surgery). The marker means typically include three markers that are arranged in a fixed and predetermined location relative to each other and are in particular mechanically linked. The markers can be passive or active markers, wherein passive markers reflect signals (e.g., waves and/or radiation) emitted in their direction, and active markers are themselves the origin of the signals (e.g., radiation and/or waves). The signals emitted by the (active or passive) markers, which can be wave signals or radiation signals, are detected by a detection device (e.g., a camera). In order to establish a position of the marker means relative to the detection means, the marker means is preferably moved to provide the detection means with various views of the marker means. On this basis, the location of the marker means relative to the detection means can be determined in a known way, in particular in a spatial reference system. Reference is made in this respect to DE 196 39 615 A1 and the corresponding U.S. Pat. No. 6,351,659, which are hereby incorporated by reference in their entirety.
Instrument calibration can lose its validity in the course of time. In particular, the geometric properties of the medical treatment device can change in the course of time. This can be caused by attrition or by being mechanically burdened once or repeatedly.